How to create a type from the union of two interfaces?

Question

Consider the following:

interface Man {
  name: string
}

interface Dog {
  breed: string
}

let manDog: Man | Dog;

Is it possible to create a type ManDog which is the equivalent of Man | Dog so that one could do this:

let manDog: ManDog;

Answer 1

You can use the type keyword (not let) to create a new type an alias for your union type.

https://www.typescriptlang.org/docs/handbook/advanced-types.html

interface Man {
  name: string
}

interface Dog {
  breed: string
}

type ManOrDog = Man | Dog;

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To use effectively these union types, don't forget to have a look on type guards, explained in the same documentation link.

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EDIT : Why did I replaced new type by alias :

A new type would be another Interface, that you could extend and so on.

You cannot create an Interface that extends the alias created by type.

Also, error messages won't show ManOrDog.

In this case, for a union type, the alias is the only way.

For an intersection type however, you would have the choice between a new type with

interface ManAndDog extends Man, Dog { }

and an alias

type ManAndDog = Man & Dog;